1. Field of the Invention
The present invention relates to processes for producing a golf ball. More particularly, the present invention relates to improvement of compression molding processes of a cover.
2. Description of the Related Art
General golf balls other than those for use in practice ranges have a core and a cover. The cover is composed of a thermoplastic resin composition. For forming a cover, an injection molding process or a compression molding process is employed.
The injection molding process is excellent in mass production capability. According to the injection molding process, the core is first retained at a center of a spherical cavity by holding pins. Next, a molten thermoplastic resin composition is injected into a gap between the cavity face and the core. Because the holding pins shift backward in a final stage of the injection, the core may deviate from the center, concurrent with the flow of the resin composition. The deviation causes unevenness in the thickness of the cover (generally referred to as “uneven wall thickness”). The air which is present in the gap between the cavity face and the core is discharged from a bent hole or a clearance of the holding pin in accordance with the influx of the resin composition. When this discharge is insufficient, defective appearance due to the residual air occurs. Accordingly, difficulties are involved in the production of a high quality golf ball by the injection molding process.
In a compression molding process, two half shells composed of a thermoplastic resin composition, and a core covered by these half shells are placed into a mold. This mold comprises an upper portion and a lower portion. By clamping this mold, the resin composition is compressed, and the excess resin composition outflows from the parting line. The air which is present between the core and the half shell is discharged from the parting line concomitant with the outflow of the resin composition.
When the resin composition outflows toward a specific direction in a converged manner in the compression molding process, uneven wall thickness is caused. In particular, when the amount of outflow is abundant, uneven outflow is liable to take place. Furthermore, the resin composition in the vicinity of the parting line is liable to outflow, whilst the resin composition which is positioned far from the parting line hardly outflows, therefore, sphericity of the golf ball may become thereby insufficient. Uneven thickness and low sphericity exert a detrimental effect on performances of the golf ball. Particularly, performances of a golf ball having a small nominal thickness of the cover are greatly influenced by the detrimental effect of the uneven thickness and low sphericity.
In Japanese Patent No. 3130946, a process for producing a golf ball in which an injection molding process and a compression molding process are combined. According to this process for the production, a preformed piece is first obtained by injecting a resin composition around a core. This preformed piece is placed into a mold, and a cover is formed by a compression molding process. This process for the production requires two steps for forming the cover.
An object of the present invention is to provide a process for the production in which a golf ball having excellent sphericity is obtained with suppressed uneven wall thickness.